This invention relates to the molecular averaging of light and heavy hydrocarbons, predominantly C2-C5 and C20+ paraffinic feedstocks.
The majority of commercially available fuels and lubes are derived from crude oil. Crude oil is in limited supply, and fuels and lubes derived from crude oil are often contaminated with sulfur and nitrogen compounds, which contribute to environmental problems such as acid rain and ground level ozone. For these reasons, efforts have focused on methods for forming fuels and lubes from feedstocks other than crude oil.
Several methods have been developed for converting natural gas to heavier hydrocarbons. One method involves converting methane to methanol, and then converting the methanol to higher molecular weight products. Another method involves converting methane to synthesis gas (xe2x80x9csyngasxe2x80x9d), a mixture of carbon monoxide and hydrogen gas, and subjecting the syngas to Fischer-Tropsch synthesis to form a mixture of heavier products.
Fischer-Tropsch chemistry tends to produce a wide range of products from methane, low molecular weight paraffins, olefins and other light hydrocarbons to heavy wax. The low molecular weight olefins can be oligomerized, and the waxes can be hydrocracked, with the products optionally isomerized, to provide hydrocarbons useful in compositions of gasoline, middle distillate fuels and lubes. The hydrocracking step requires the use of hydrogen and also produces undesirable, low value cracking products such as C1-C5 paraffins. It is advantageous if gasoline, middle distillate fuels and lubes can be made from the heavy wax without producing the undesirable low-value light cracking products. It is additionally advantageous if these undesirable, low-value, light hydrocarbons can be used to make gasoline, middle distillate fuels and lubes. The present invention provides such methods.
In its broadest aspect, the present invention is directed to an integrated process for producing gasoline and middle distillate fuels and lube oils from light paraffin-containing fractions and C20+ paraffinic fractions. The process involves combining an light paraffin-containing fraction with a C20+ paraffin containing feedstock and subjecting the paraffins to conditions of molecular averaging. During the course of this reaction, the feedstock is simultaneously subjected to dehydrogenation, olefin cross-metathesis, and rehydrogenation. The molecular averaging reaction provides a product stream that includes paraffins having molecular weights between those of the light and heavy feedstocks.
The product stream includes a fraction rich in paraffins the molecular weights of which are between those of the light and heavy paraffin feedstocks, plus some unconverted feeds. Preferably, the product stream can be distilled to provide a C2-5 fraction, a fraction suitable for use in preparing gasoline, middle distillate fuels and lubes, and a C20+ fraction. The C2-5 and C20+ fractions can be recycled to extinction. The desired molecular averaging product fraction can be isomerized to improve the octane value, in the case of gasoline, or to improve the pour point, in the case of diesel fuel or lubes.
A useful light paraffin-containing feedstock containing C2-5 paraffins for the reaction can be obtained by subjecting natural gas well effluent to demethanization. Such a light paraffin feedstock can also be obtained from the products of Fischer-Tropsch synthesis. Products of Fischer-Tropsch synthesis are also a useful source of heavy hydrocarbon feedstock.
Methane does not participate in the reaction (it cannot be dehydrogenated to form an olefin) but does dilute the reactants and, in one embodiment, is removed from the feedstock.
In another embodiment, natural gas, containing methane and C2-5 hydrocarbons, is used in the molecular averaging reaction, to decrease the concentration of the C2-5 hydrocarbons (hence enriching the methane in the low-boiling fraction) in the natural gas and providing hydrocarbons in the gasoline, middle distillate fuel and lube ranges.
Due to the nature of the molecular averaging chemistry, the feedstock cannot include appreciable amounts (i.e., amounts that would adversely affect the catalyst used for molecular averaging) of hydrogen, olefins, alkynes, thiols, amines, water, air, oxygenates or cycloparaffins.
Hydrogen, water, air and, optionally, methane, can be removed from feedstocks using conventional methodology, for example, using demethanizer columns. Methods for removing sulfur, oxygenates and nitrogen compounds are well known, and generally involve hydrotreating the feedstock. Methods for removing cyclic compounds are also known in the art and generally involve adsorption and separation by molecular sieves.
Refinery waste gas predominantly includes hydrogen and C1-5 paraffins, but may include small amounts of olefins and alkynes, as well as heteroatom-containing impurities. Natural gas predominantly includes C1-5 paraffins, but may include sulfur and nitrogen impurities. Cracked gas feedstreams predominantly include hydrogen and C1-6 paraffins, olefins, alkynes and sulfur and nitrogen impurities. The hydrogen, heteroatom-containing impurities, and, optionally, the methane from these feedstocks are removed, and any olefins and/or alkynes hydrogenated, before the feedstocks are used in the processes described herein.